The present invention relates to a method and an apparatus for determining the concentration of gases in a mixture of gases by means of a gas sensor having a solid electrolyte.
A plurality of embodiments of electrochemical gas sensors is based on watery electrolytes or polymeric solid electrolytes (U.S. Pat. No. 4,227,984). The materials as per the latter are water containing proton conductors.
The use of a watery or water containing electrolyte limits severely the field of use for the sensors. Because the water in the electrolyte evaporates even at temperatures below 100.degree. C. and the, conductivity of the electrolyte drops sharply.
Another drawback of these gas sensors using a water containing electrolyte is the considerable expenditure needed for miniaturization and realization of a compact design ( British patent application 22 28 327).
The measuring effect of such sensors is based on the electrochemical oxidation or reduction of the gas component to be detected and on the formation of protons or water at a measuring or a counter electrode. The selectivity of these sensors is thus limited by the oxidation or the reduction potential of the gas species to be detected.
It is known that the concentrations of gases in a mixture can be measured by means of ion-conductive materials provided certain conditions are met ( see H. Rickerr "solid ion conductors principles and application" in a journal titled Angew. Chem. (applied chemistry) vol. 90 pages 38-48, 1978). For that purpose a galvanic chain is constructed being comprised of two electrodes and a solid electrolyte wherein one electrode, the so called measuring electrode, is exposed to the gas whose composition is unknown and the other electrode is held to constant conditions through appropriate features. In order to obtain a suitable measuring signal essentially two measuring principles are used:
a) measuring the voltage between the electrodes (potentiometer measurement) without any current flow if at all possible or PA1 b) measuring the current between the electrodes at a given voltage (potentiostatic or amperic measurement) PA1 R is the universal gas constant PA1 T is the absolute temperature PA1 F is the Faraday constant PA1 n is the number of electrons that are transferred PA1 Pgas is the partial pressure of the gas to be measured. PA1 The solubility of the gas component to be detected is not present for a solid electrolyte. Thus the oxidation or reduction of an electrically neutral gas particle is not possible directly from the gas phase combined with an electron transfer at an electrode for maintaining the charge. In order to accomplish this an ion has to Be produced from the electrically neutral gas particle which requires a second charge carrier of opposite polarity in order to stabilize the product. This kind of charge carder is available in the above mentioned electrolyte of the conventional electrochemical gas sensor as OH.sup.- or H.sub.3 O.sup.+ either as ions or as impurity ions. PA1 The electro-active ions of the solid electrolyte can take up this task at the three phase boundary solid electrolyte/electrode/gas only if they emerge from the solid electrolyte and combines with the gas particles. PA1 Upon using solid electrolytes which do not have any solubility or mobility for any of the participating gas components and reaction products a redox reaction with the gas to be detected has to take place at the three phase boundary electrolyte/electrode/gas. PA1 In addition to that reaction a compound has to result which is a metal salt if at the given solid electrolyte the electro-active ions are metal ions. The metal salt will be precipitated on the electrode at the particular polarization and upon reversing the voltage it can be decomposed. The amount of metal salt that forms is generally linearly dependent upon the partial pressure of the participating gas, unless the ion current is limited at the electrode itself. PA1 Contrary to the electrolyte that has a solubility for the electrochemical reaction product in the case of a solid cat-ionic electrolyte the reaction product has to be electrochemically decomposed.
A known example for the principle a) is the lambda probe using zirconium dioxide ZrO.sub.2 as solid electrolyte. This probe is used for measuring the concentration of oxygen. The counter electrode is exposed to the relatively constant partial pressure of oxygen in air.
Such measurements can also be conducted when the solid electrolyte cannot directly transmit the ions of the gas to be detected (German patent 29 26 172 C2). Furthermore it is not always necessary to expose the counter electrode to a reference gas, but the latter electrode may be comprised of a material which contains the ions of the solid electrolyte either as a chemical compound or in elemental form. The partial gas pressure is determined by means of such a galvanic chain and uses the chemical reaction of the gas to be detected with an electric-active ion of the solid electrolyte. For this an electric voltage (EMF) is measured between the reference and the measuring electrodes, a reaction takes place at the latter. The higher the partial pressure of the gas reacting with the ion the larger is the electric voltage.
The dependency of the partial pressure is determined by the Nernst equation: EQU EMF=E.sub.0 +(R*T)/(n*F) * lognat (p.sub.gas)+E.sub.r +Z
Herein,
The parameter E.sub.r contains the portion of the counter electrode that is independent from the partial pressure. and the parameter Z combines all terms that are attributable to other gas components. The quantity E.sub.0 contains the Gibbs reaction enthalpy to be calculated for the reaction of the gas with the ions of the solid electrolyte by means of the equation G/(n*F)=-E.sub.0.
The quantity G informs about the thermodynamic stability of a compound. The ions of the solid electrolyte may combine with different gases in differently stabile compositions.
For example, sodium ions can be transferred through the solid electrolyte Na-.beta.-alumina and together with sulfur-dioxide in the presence of oxygen it will form sodium sulfate or together with carbon-dioxide it will form sodium carbonate. In the most simple case the compound with the largest reaction enthalpy will form under conditions given by the gas mixture, in the present example this is the case for sodium sulfate.
FIG. 1 illustrates the temperature dependency of the EMF in principle and as determined by the Nernst equation for the formation of the metal salts sodium carbonate Na.sub.2 CO.sub.3 and sodium nitrate NaNO.sub.3. Herein the upper curve illustrates in each instance the situation for a high partial pressure and the lower curve represents the EMF for a low partial pressure of CO.sub.2 and NO.sub.2. Thus, there exists a temperature range and a partial pressure range wherein an EMF is produced by a Na chain and for a particular partial pressure with which a particular CO.sub.2 partial pressure can be associated.
A separate potentiometric determination of partial gas pressure of either NO.sub.2 or CO.sub.2, both gases being present, by means of a Na chain is thus possible only under very limited conditions. The thermodynamic calculations for other metal salts such as Ag salts establish likewise temperature and partial pressure ranges in which several reactions run in parallel in a competing fashion because their reaction enthalpy and EMF values are identical. Consequently the EMF measurement is inconclusive as to the partial gas pressure of interest.
Some kind of reactions are preferred kinetically over others in addition to the thermodynamically conditioned cross-sensitivity. For example 2 particle reactions are more probable than 3 or 4 particle reactions. Additional reaction impediments may cause chemical reactions having a lower reaction enthalpy to occur ahead of those with higher reaction enthalpy. For these reasons and in the case of several gas components being present, the EMF values cannot generally be correlated with the actual partial pressure of a particular component to be detected.
In order to still obtain the required selectivity it has been suggested to embed the measuring electrode in the metal salt that results from the ions in the solid electrolyte and the component to be detected (European patent 0 182 921 B1). Large time constants following a change in concentration, drift of the measuring signal and limiting measurement to but one component by that one sensor are disadvantages of this arrangement.
Another approach for selectivity of electrochemical gas sensors used cyclic voltammetry. The gas species to be detected passes through a membrane into the electrolyte in this kind of gas sensors.
The electrolyte is comprised of a substance which has a good solubility of the gas to be detected. As a voltage is applied to the electrodes the gas is oxidized or reduced. The reaction products are also dissolved in the electrolyte and are then removed from the measuring electrode.
Different kinds of gas species which can be dissolved in the electrolyte can be electrochemically changed at different voltages. If the voltage is varied in time, e.g. if a triangularly shaped signal is applied and the current is then measured, current-voltage curves can be ascertained which show a higher current at the redox-potential of the different gas species (see e.g. J. D. Zook and H. V. Venkastasetty "Non aqueous Electrochemical Gas Sensors" in Transducers 1985 International conference on solid state Sensors and Actuators, pages 326 to 329; and H.Gayet and L. T. Yu "Application of linear Potential sweep voltammetry to make Gas Captors" in Sensors and Actuators vol. 15 1988 p.387-398; and J. Bergman "The voltammetry of some Oxidizing and Reducing Toxic Gasses direct from the Gas Phase at Gold and Platinum Metallised-membrane Electrodes in Acid and Alkali" in J. Eleectroanal. Chem, vol. 157 1983 p. 59-73).
In order to apply this method to solid electrolytes which, contrary to e.g. oxygen conducting zirconium dioxide, will not transfer the gas to be detected, the following differences are important:
In addition to high selectivity further measurement technological requirements are, a stable and reproducible measuring signal, a fast response time to changes in the concentration as well as a long use life. In order to fulfill these requirements in the present case that the measuring electrode must be regenerated completely during each cycle. This means that the metal salt which was formed must be completely decomposed independently from the actually existing gas concentration. Otherwise the metal salt layer will grow uncontrolled during operation which changes the response time, the magnitude of the measuring signal and the sensitivity of the gas-sensor.
It was found that the decomposing of the metal salt is not a necessarily useful process for compensating the charges of formation and decomposing. Since the transfer number for electronic charge carriers in the solid electrolyte as used here (unlike in the case for liquidous electrolytes) differs from zero an electronic contribution to the process disturbs the equilibrium.
It is not sufficient in the case of solid electrolyte sensors to compensate the changes that occur on the measuring electrode just by way of calculations as is suggested in German patent application 30 26 824 disclosing an electrochemical sensor for determining the oxygen content in blood. This is so owing to the uncontrolled growth of the metal salt layer e.g. through the formation of a composition with a particular gas component would reduce the sensitivity of another gas component. Presently investigations have shown that the regeneration of the measuring electrode is obtained whenever the layer of the metal salt does not exceed an equivalent charge density of 2*z mAs/cm.sup.2, herein z is the number of charges per ion.